fix(ns): root-qualify PHP builtins after PSR-4 move

Namespaced classes resolved DateTimeImmutable etc. under AKSRT\*
(e.g. Credits\DateTimeImmutable). Prefix with \ globally.

src/AKSRT/Credits/Invite.php

@@ -238,7 +238,7 @@ class Invite {
         $log_table = \AKSRT\Core\DBMigrator::table('credit_log');
         if (!$log_table) return true;
 
-        $today_utc = (new DateTimeImmutable('today', wp_timezone()))->setTimezone(new DateTimeZone('UTC'))->format('Y-m-d H:i:s');
+        $today_utc = (new \DateTimeImmutable('today', wp_timezone()))->setTimezone(new \DateTimeZone('UTC'))->format('Y-m-d H:i:s');
         $count = (int) $wpdb->get_var($wpdb->prepare(
             "SELECT COUNT(*) FROM {$log_table} WHERE reason = 'invite' AND ip = %s AND created_at >= %s",
             $ip,